The present invention relates to silver halide emulsions useful in the field of photography and silver halide light sensitive color photographic materials using the same, in particular, to a silver halide color photographic material exhibiting superiority in sensitivity, graininess, pressure characteristics, fogging, radiation fogging, aging stability, latent image keeping stability, latent image variation depending of temperature and humidity and productivity.
Recently, along with the increased popularity of compact cameras, auto-focus single-lens reflex cameras and single-use cameras, a silver halide light sensitive color photographic material exhibiting further enhanced sensitivity and image quality has been desired by the public. Requirement for improvements in performance of silver halide photographic emulsions becomes severe and high level requirements for enhanced sensitivity, superior graininess and superior sharpness have been sought.
In response to such requirements, U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4,433,048, 4,414,306 and 4,459,353 disclose a technique of using tabular silver halide grains (hereinafter, also referred to as tabular grains), exhibiting advantages such as enhancement of sensitivity including enhancement of efficiency of spectral sensitivity with a sensitizing dye, an improvement in sensitivity/graininess, an enhancement of sharpness due to a specific optical property of the tabular grains and enhanced covering power. However, this technique is still insufficient in response to recent higher level requirements and still further enhanced performance is desired.
Relating to the trend of enhanced sensitivity and enhanced image quality, requirements for enhancement of pressure characteristics continues to increase. There have been made attempts to improve pressure characteristics by various means. It is generally regarded that a technique of enhancing pressure resistance of silver halide grains themselves is more preferred in terms of practical use and is considered to be more effective than a technique of adding additives such as an addition of a plasticizer.
On the other hand, enhancing uniformity among silver halide grains contained in a silver halide emulsion is also important to enhance performance of a silver halide photographic material using a silver halide emulsion. There are known various techniques giving attention to the uniformity among silver halide grains. Examples thereof include a technique concerning tabular silver halide grains uniform in grain size, as described in JP-A Nos. 1-213637, 5-173268 and 6-202258 (hereinafter, the term, JP-A refers to an examined and published Japanese Patent Application), a technique concerning tabular silver halide grains uniform being in grain thickness described in Japanese Patent Application No. 9-218567, a technique concerning tabular silver halide grains uniform being in grain size and thickness described in Japanese Patent Application No. 8-166040, and a technique concerning uniformity of dislocation lines in tabular silver halide grains described in Japanese Patent Application No. 8-149163. Further, JP-A No. 2-256043 describes a technique concerning silver halide grains uniform being in an average iodide content and WO89/06830 describes a technique concerning microscopic uniformity of halide composition within the silver halide grain.
With regard to the technique giving attention to the twin plane of tabular silver halide grains, JP-A 63-163451 describes a silver halide emulsion comprised of tabular grains having a grain diameter of 0.15 xcexcm or more and an average aspect ratio of 8 or more and exhibiting a longest spacing between parallel twin planes to grain thickness of 5 or more, WO91/18320 describes a silver halide tabular grain emulsion having an average grain diameter of at least 0.6 xcexcm and a spacing between parallel twin planes of 0.011 xcexcm or less, JP-A 5-249585 describes a silver halide tabular grain emulsion having an aspect ratio of less than 4 and exhibiting a value of a grain thickness (T) divided by a spacing between twin planes (S) of more than 15, JP-A 8-110605 describes a silver halide emulsion comprised of tabular grains having an average spacing between twin planes (d0) of 0.025 xcexcm or less within the range of 0.8d0 to 1.2d0 xcexcm, accounting for at least 75% of the total grain projected area, and JP-A 9-203985 describes a silver halide tabular grain emulsion having a spacing between twin planes of 0.025 xcexcm or less and a variation coefficient of the spacing between twin planes of 40% or less, each of which shows improvements in sensitivity, graininess and pressure resistance. In any of these techniques, however, non-uniformity among silver halide grains is still pronounced and a further improvement in photographic performance is still remained.
U.S. Pat. No. 4,956,269 describes a technique of introducing dislocation lines into tabular silver halide grains to enhance sensitivity. It is commonly known that application of pressure to the silver halide grain results in fogging or desensitization and there are also problems such that the dislocation line-introduced grains were markedly desensitized upon application of pressure. JP-A 3-189642 describes a silver halide emulsion accounted for by tabular grains having an aspect ratio of 2 or more and 10 or more dislocation lines in the fringe portion of the grain, exhibiting monodisperse grain size distribution. However, marked desensitization caused by introduction of dislocation lines cannot be overcome by such a technique.
Further, one of the problems occurring along with enhanced sensitivity is an aging fog increase caused by a slight amount of radiation existing in the natural ambient environment, accompanied with reduced sensitivity and deteriorated graininess. As is well known, radiation having higher energy than visible light used in exposure of photographic materials causes interaction with substances, generating a number of secondary electrons. Interaction of the radiation with silver halide grains generates a number of secondary electrons for a very short period of time within the grain to form plural development-initiation points within the grain, even in low-exposure areas, so that a photographic material exposed to natural radiation causes more marked deterioration in graininess than that caused by fogging due to heat or humidity. Fog-increase caused by radiation is generally proportional to silver coverage of the photographic material so that reduction of silver coverage has been thought to be effective to reduce radiation fogging. However, since reduction of silver coverage results in reduced sensitivity, it is limited to achieve both enhanced sensitivity and reduced radiation fogging in a high-speed photographic material having higher silver coverage relative to a low-speed photographic material. Cited as a technique for reducing silver coverage without reducing sensitivity is the use of tabular grains, each which has a larger surface area relative to an identical volume. However, the use of tabular grains alone is insufficient to reduce radiation fogging. Further, a method for reducing radiation fogging by the use of a specific color developing agent is known, as described in JP-A 4-337737 but reduced sensitivity along with reduction of silver coverage is also shown therein.
Ultrafiltration is employed as a method for concentrating the volume of the reaction mixture (silver halide emulsion) in the process of emulsion making, as described in JP-B No. 59-43727 (hereinafter, the term, JP-B refers to published Japanese Patent) and JP-A 3-140946. However, in these disclosure are no suggestion with respect to tabular grains or a monodisperse silver halide tabular grain emulsion. JP-A 6-67326 describes a method in which ultrafiltration is applied during the stage of preparing a silver halide tabular grain emulsion to concentrate the reaction mixture, whereby not only the yield is enhanced but also tabular grains having an intermediate aspect ratio of 2 to 8. were obtained. In this disclosure, taking advantage of employing the fact that the aspect ration of silver halide grains systematically decreases along with concentration, a tabular grain emulsion having a high aspect ratio was concentrated by ultrafiltration to obtain an intermediate aspect ratio. However, silver halide emulsions exemplified in this disclosure, including comparative and inventive emulsions exhibiting 30% or more of a variation coefficient of grain size distribution, based on volume equivalent diameter and uniformity in shape or structure of the tabular grain, as one feature of the present invention was not obtained. Further, enhancement of uniformity of the grains, that is, precise control of grain thickness distribution, the distance between the major face and the closest twin plane, or spacing between twin planes is not intended in this disclosure.
Accordingly, precise control of silver halide grains is limited in the prior art, in which enhanced performance achieved by enhancing uniformity of the grains cannot be expected, and development of further prominent technique is desired.
In view of the foregoing, it is an object of the present invention to provide a silver halide emulsion having enhanced sensitivity and superior graininess and exhibiting superiority in pressure characteristics, radiation fogging and aging stability and a silver halide color photographic light sensitive material using the same.
The object of the present invention can be accomplished by the following means:
1. a silver halide emulsion comprising silver halide grains, wherein a variation coefficient of grain diameter of the whole silver halide grains is not more than 25%, wherein at least 50% of the total grain projected area is accounted for by tabular grains having two twin planes and an aspect ratio of not less than 6 and meeting the following requirement (1):
1.0xe2x89xa6b/axe2x89xa61.3xe2x80x83xe2x80x83(1)
wherein, of a distance between one major face and the twin plane closest thereto, and a distance between the other major face and the twin plane closest thereto, xe2x80x9caxe2x80x9d is a shorter distance and xe2x80x9cbxe2x80x9d is a longer distance;
2. the silver halide emulsion as described in 1 above, wherein at least 50% by number of the tabular grains is accounted for by tabular grains having dislocation lines of 10 or more per grain which are localized in the peripheral region of the grain and an area ratio of the region occupied by the dislocation lines being 5 to 40% of the major face and a variation coefficient of the area ratio being not more than 30%;
3. the silver halide emulsion as described in 1 above, wherein a variation coefficient of grain thickness of the silver halide grains contained in the emulsion is not more than 30%;
4. the silver halide emulsion as described in 2 above, wherein a variation coefficient of grain thickness of the silver halide grains contained in the emulsion is not more than 30%;
5. the silver halide emulsion as described in 1 above, wherein the tabular grains have a mean spacing between twin planes of 0.013 to 0.017 xcexcm;
6. the silver halide emulsion as described in 5 above, wherein the tabular grains have a mean grain thickness of 0.05 to 1.5 xcexcm;
7. the silver halide emulsion as described in 1 above, wherein a variation coefficient of spacing between twin planes of the tabular grains is not more than 25%;
8. the silver halide emulsion as described in 5 above, wherein a variation coefficient of spacing between twin planes of the tabular grains is not more than 25%;
9. the silver halide emulsion as described in 1 above, wherein at least 50% by number of the tabular grains is accounted for by hexagonal tabular grains exhibiting a mean value of a ratio of maximum edge length to a minimum edge length of not more than 1.5, and a variation coefficient of the ratio of maximum edge length to a minimum edge length being not more than 25%;
10. the silver halide emulsion as described in 1 above, wherein a variation coefficient of iodide content distribution among the silver halide grains contained in the emulsion is not more than 25%;.
11. the silver halide emulsion as described in 1 above, wherein at least 40% by number of the tabular grains is accounted for by tabular grains having dislocation lines in the central and peripheral regions of the major face;
12. the silver halide emulsion as described in 1 above, wherein the silver halide emulsion is prepared by a process comprising nucleation and grain growth, wherein a part of water of the emulsion is removed through ultrafiltration over a period of after completion the nucleation and before completion of the grain growth to maintain the intergrain distance between silver halide grain or to decrease the intergrain distance;
13. a silver halide color photographic material comprising a support having thereon a red-sensitive layer, a green-sensitive layer, a blue-sensitive layer and a light-insensitive layer, wherein at least one of the red-, green- and blue-sensitive layers comprises the silver halide emulsion as described in 1 above.